This invention relates to exhaust nozzles for directing gas turbine exhaust gas into the atmosphere to propel an airplane or other vehicle. More particularly, this invention relates to a variable geometry convergent-divergent exhaust nozzle configured for providing optimum thrust over a wide range of flight conditions including operation of an aircraft at subsonic and supersonic speeds.
It is known that maximum thrust and operating efficiency of a gas turbine engine that supplies propulsive thrust is obtained when the engine exhaust effluent is directed through an exhaust nozzle which controls the expansion of the exhaust gases since controlled expansion of the high temperature, high pressure gases supplied by the gas turbine engine increases the particle velocity of the exhaust effluent and hence increases the momentum of the thrust exhaust producing stream. In this respect, maximum operating efficiency is generally achieved when the nozzle is configured to exit the exhaust stream at substantially the same pressure as that of the surrounding ambient atmosphere.
When an aircraft operates both at subsonic and at supersonic speeds, the exhaust nozzle pressure ratio, i.e., the ratio of the total fluid pressure upstream of the nozzle to the ambient atmospheric pressure, varies over a substantial range. In particular, under subsonic flight conditions the nozzle pressure ratio is relatively small and a nozzle having a convergent shape provides the desired expansion characteristics. On the other hand, under supersonic flight conditions, the nozzle pressure ratio is quite high and proper expansion of the exhaust effluent is effected by an exhaust nozzle having a convergent portion followed by a divergent portion, which type of exhaust nozzle is generally referred to as a convergent-divergent exhaust nozzle. Moreover, fairly substantial variations in pressure ratio results from various engine throttle settings and, in some cases, also results from "ram effect" when an increased amount of air is effectively forced through the engine as the aircraft moves through the atmosphere at high speed. Because of these factors a fixed geometry exhaust nozzle is often not satisfactory. Accordingly many attempts have been made to design variable geometry exhaust nozzles which are operable to exit the engine exhaust into the ambient atmosphere at approximately the same pressure as that of the atmosphere during all flight regimes.
In general, such prior art attempts have included variable geometry plugs which extend rearwardly relative to the flow of exhaust gases and which are supported within an outer housing or duct of fixed geometry; a variable geometry outer housing which may or may not include a rearwardly extending central plug of fixed geometry; and, the combination of a variable geometry outer housing and a variable geometry plug. Since the variable geometry exhaust nozzles include means for varying both the geometry of the outer duct and the geometry of the plug permit control over the area of an annular throat region which is formed between the maximum diameter region of the plug and the minimum diameter region of the outer housing and permit control over the nozzle exit area, the latter type of variable geometry nozzles is generally more desirable than types in which only the outer housing or only the plug geometry can be controlled.
Although a variety of exhaust nozzles in which both the plug and outer housing are of variable geometry have been proposed, such prior art nozzles have not simultaneously met all of the necessary design criteria. For example, to minimize drag, such an exhaust nozzle must be containable within the conventional housing arrangement of a gas turbine engine installation and, to effect overall cost efficiency in an aircraft, must be of acceptable weight. Further, to provide reliable operation and economy of fabrication, such an exhaust nozzle and the associated operating mechanism must not be unduly complex.
Beyond failing to adequately comply with the basic requirements, the prior art variable geometry exhaust nozzles exhibit other drawbacks and disadvantages. First, these devices generally have not been configured for operating in conjunction with thrust reverser apparatus that is conventionally employed in a thrust producing gas turbine engine installation. Thus, additional apparatus must often be included to control the engine exhaust gases during thrust reversal operation, thereby generally increasing the weight, cost and complexity of the overall engine installation. Further, because of the relatively high pressure, high velocity flow within the exhaust nozzle, the apparatus which operates the variable outer housing of the exhaust nozzle is subjected to substantial forces that are exerted in the outward radial direction. To withstand these forces and thereby provide proper pressure containment, the prior art operating apparatus has generally been heavy and relatively slow in operating speed. Such limitations in operating speed do not permit rapid and precise changes in exhaust nozzle geometry that can be desirable under certain engine operating conditions. In particular, relatively rapid changes in pressure of either a cyclic or sporadic nature can occur under supersonic and transonic flight conditions. Since such pressure changes not only cause at least a temporary decrease in operating efficiency, but can cause pressure disturbances within the engine that result in damage to the engine, it is highly desirable to rapidly effect an appropriate decrease in nozzle throat area as such pressure changes occur. Prior art variable geometry exhaust nozzles have not been constructed in a manner which permits rapid and precise modulation of the nozzle throat area under such conditions.
Accordingly it is an object of this invention to provide a variable geometry exhaust nozzle configured for use in a gas turbine engine installation, such exhaust nozzle being operable between a convergent configuration and a convergent-divergent configuration.
It is another object of this invention to provide a gas turbine engine exhaust nozzle of the above described type wherein the geometry of both the exhaust nozzle outer housing and the geometry of the rearwardly extending tail plug can be continuously varied, either independently or simultaneously to provide a wide range of nozzle throat areas and exhaust nozzle exit areas.
It is still another object of this invention to provide a variable geometry exhaust nozzle of the above described type wherein the nozzle throat area can be precisely and rapidly controlled in response to rapid changes in exhaust nozzle pressure ratio while simultaneously providing an exhaust nozzle structured for containing the relatively high pressure engine exhaust effluent.
It is yet another object of this invention to provide an exhaust nozzle of the above described type that is relatively light in weight, containable within a region of relatively low volume, and of relatively low structural complexity.